The SSTO is not intended to return to Earth. Since rocket tanks hold pressurized propellent, and an orbiting greenhouse would be a gas pressurized transparent tank with plants inside. Why not make a single stage rocket, out of a hard, transparent material, like Zirconia, or Alumina, designed to be used as a greenhouse in orbit? In orbit, stick some gases, plants and maybe a person inside. Use hydrogen as a propellant. We want the hydrogen tank to be enormous. Stick a bunch across the inner solar system to resting places.

Yes, I know there are all sorts of properties of materials, fractures, heat control to worry about, etc.

It would have to be truly enormous. Reuse of propellant volume IS a great idea (but not a new one unfortunately). However, present materials and propulsion efficiencies probably make this not practical.

Alumina is about 50% more dense (i.e. heavy, for a given volume) than aluminium, and much more brittle, so a strong enough rocket tank made out of it would probably be way too heavy to get off of the ground.

A transparent version of Bigelow's inflatables seems to me to be a more workable idea. Transparent plastics are nothing new, and for example polyethylene is a relatively good radiation shield as well. Make it a double wall with water inside for more shielding, or spin up the whole thing and leave out the inner wall so that you can do hydroponics in the shielding water.

There's another thing in that plant photosynthesis is not a continuous process: the leaf takes up a photon, and is then "busy" processing for a while. Philips Lighting is working (maybe it's already on the market actually, I don't know) on special LED lights that "blink" extremely quickly, tuned to the duty cycle of plants. So, while the plant is busy processing a packet of light energy, the LED is turned off to save power. If the duty cycle is low enough (I don't know how big it is off the top of my head) and/or the efficiency of your solar cells high enough, then just putting such lights inside and coating the outside of your space ship with solar cells may be even more efficient than transparent walls.

_________________Say, can you feel the thunder in the air? Just like the moment ’fore it hits – then it’s everywhereWhat is this spell we’re under, do you care? The might to rise above it is now within your sphereMachinae Supremacy – Sid Icarus

It would have to be truly enormous. Reuse of propellant volume IS a great idea (but not a new one unfortunately). However, present materials and propulsion efficiencies probably make this not practical.

Alumina (sapphire) is a very strong material, but it might indeed be too brittle. I figured the propellant tank space stations weren't researched because of lack of interest from NASA management & Congress. I would love to see a professional study done on propellant tank space stations.

_________________Say, can you feel the thunder in the air? Just like the moment ’fore it hits – then it’s everywhereWhat is this spell we’re under, do you care? The might to rise above it is now within your sphereMachinae Supremacy – Sid Icarus

Just by taking the recommendations of that group, especially after the use of the lithium alloy, we could have the equivalent of a small city in orbit today.

Of course the complication and technical reason it wasn't done, was because, beyond bureaucratic inertia and politics, was that the orbiters went up into all sorts of orbits depending on the mission and what they were delivering. If the ETs were being saved, they would always have to launch to the same orbit or have some kind of tug that could match orbits, retrieving it, and then take it to the station/collection point. Too much cost to a project that was already over budget. Much easier to spend billions more for less volume in a discreetly separate project (SS Freedom/ISS).

Even diamond would probably be too brittle, but maybe we're thinking about this the wrong way. First of all, why does it need to be transparent? Translucent might be just as better, with diffusion, and a lot easier to acceive with composites, which do seem to do a lot better in pressure vessels.

Also, as we still have yet to build a true OStEO (One Stage to Earth Orbit, my personal acronym,) launcher, might we be getting a little ahead of ourselves in trying to design aplications for it? Without one I can point to, I can't say what it would look like, so would be hard pressed to decide what could be built with one, and how.

My guess is, it won't be acheivable by stacking a big honking tank on top of a motor, and hoping it will have the proper mass fraction at all altitudes. We've tried that already, and besides, that's not 1 tank, unless you're proposing a monopropellant rocket, which I havent seen at those thrust levels either.

_________________"You can't have everything, where would you put it?" -Steven Wright.